# Configure alerts
Netdata's health watchdog is highly configurable, with support for dynamic thresholds, hysteresis, alert templates, and
more. You can tweak any of the existing alerts based on your infrastructure's topology or specific monitoring needs, or
create new entities.
You can use health alerts in conjunction with any of Netdata's [collectors](/src/collectors/README.md) (see
the [supported collector list](/src/collectors/COLLECTORS.md)) to monitor the health of your systems, containers, and
applications in real time.
While you can see active alerts both on the local dashboard and Netdata Cloud, all health alerts are configured _per
node_ via individual Netdata Agents. If you want to deploy a new alert across your
[infrastructure](/docs/netdata-cloud/organize-your-infrastructure-invite-your-team.md), you must configure each node with the same health configuration
files.
## Reload health configuration
You do not need to restart the Netdata Agent between changes to health configuration files, such as specific health entities. Instead, you can use `netdatacli` and the `reload-health` option to prevent gaps in metrics collection.
```bash
sudo netdatacli reload-health
```
If `netdatacli` doesn't work on your system, send a `SIGUSR2` signal to the daemon, which reloads health configuration without restarting the entire process.
```bash
killall -USR2 netdata
```
## Edit health configuration files
You can configure the Agent's health watchdog service by editing files in two locations:
- The `[health]` section in `netdata.conf`. By editing the daemon's behavior, you can disable health monitoring
altogether, run health checks more or less often, and more. See
[daemon configuration](/src/daemon/config/README.md#health-section-options) for a table of
all the available settings, their default values, and what they control.
- The individual `.conf` files in `health.d/`. These health entity files are organized by the type of metric they are
performing calculations on or their associated collector. You should edit these files using the `edit-config`
script. For example: `sudo ./edit-config health.d/cpu.conf`.
Navigate to your [Netdata config directory](/docs/netdata-agent/configuration/README.md) and
use `edit-config` to make changes to any of these files.
### Edit individual alerts
For example, to edit the `cpu.conf` health configuration file, run:
```bash
sudo ./edit-config health.d/cpu.conf
```
Each health configuration file contains one or more health _entities_, which always begin with `alarm:` or `template:`.
For example, here is the first health entity in `health.d/cpu.conf`:
```yaml
template: 10min_cpu_usage
on: system.cpu
class: Utilization
type: System
component: CPU
lookup: average -10m unaligned of user,system,softirq,irq,guest
units: %
every: 1m
warn: $this > (($status >= $WARNING) ? (75) : (85))
crit: $this > (($status == $CRITICAL) ? (85) : (95))
delay: down 15m multiplier 1.5 max 1h
summary: CPU utilization
info: Average cpu utilization for the last 10 minutes (excluding iowait, nice and steal)
to: sysadmin
```
To tune this alert to trigger warning and critical alerts at a lower CPU utilization, change the `warn` and `crit` lines
to the values of your choosing. For example:
```yaml
warn: $this > (($status >= $WARNING) ? (60) : (75))
crit: $this > (($status == $CRITICAL) ? (75) : (85))
```
Save the file and [reload Netdata's health configuration](#reload-health-configuration) to apply your changes.
## Disable or silence alerts
Alerts and notifications can be disabled permanently via configuration changes, or temporarily, via the
[health management API](/src/web/api/health/README.md). The
available options are described below.
### Disable all alerts
In the `netdata.conf` `[health]` section, set `enabled` to `no`, and restart the agent.
### Disable some alerts
In the `netdata.conf` `[health]` section, set `enabled alarms` to a
[simple pattern](https://github.com/netdata/netdata/edit/master/src/libnetdata/simple_pattern/README.md) that
excludes one or more alerts. e.g. `enabled alarms = !oom_kill *` will load all alerts except `oom_kill`.
You can also [edit the file where the alert is defined](#edit-individual-alerts), comment out its definition,
and [reload Netdata's health configuration](#reload-health-configuration).
### Silence an individual alert
You can stop receiving notification for an individual alert by [changing](#edit-individual-alerts) the `to:` line to `silent`.
```yaml
to: silent
```
This action requires that you [reload Netdata's health configuration](#reload-health-configuration).
### Temporarily disable alerts at runtime
When you need to frequently disable all or some alerts from triggering during certain times (for instance
when running backups) you can use the
[health management API](/src/web/api/health/README.md).
The API allows you to issue commands to control the health engine's behavior without changing configuration,
or restarting the agent.
### Temporarily silence notifications at runtime
If you want health checks to keep running and alerts to keep getting triggered, but notifications to be
suppressed temporarily, you can use the
[health management API](/src/web/api/health/README.md).
The API allows you to issue commands to control the health engine's behavior without changing configuration,
or restarting the agent.
## Write a new health entity
While tuning existing alerts may work in some cases, you may need to write entirely new health entities based on how
your systems, containers, and applications work.
Read the [health entity reference](#health-entity-reference) for a full listing of the format,
syntax, and functionality of health entities.
To write a new health entity into a new file, navigate to your [Netdata config directory](/docs/netdata-agent/configuration/README.md),
then use `touch` to create a new file in the `health.d/` directory. Use `edit-config` to start editing the file.
As an example, let's create a `ram-usage.conf` file.
```bash
sudo touch health.d/ram-usage.conf
sudo ./edit-config health.d/ram-usage.conf
```
For example, here is a health entity that triggers a warning alert when a node's RAM usage rises above 80%, and a
critical alert above 90%:
```yaml
alarm: ram_usage
on: system.ram
lookup: average -1m percentage of used
units: %
every: 1m
warn: $this > 80
crit: $this > 90
info: The percentage of RAM being used by the system.
```
Let's look into each of the lines to see how they create a working health entity.
- `alarm`: The name for your new entity. The name needs to follow these requirements:
- Any alphabet letter or number.
- The symbols `.` and `_`.
- Cannot be `chart name`, `dimension name`, `family name`, or `chart variable names`.
- `on`: Which chart the entity listens to.
- `lookup`: Which metrics the alert monitors, the duration of time to monitor, and how to process the metrics into a
usable format.
- `average`: Calculate the average of all the metrics collected.
- `-1m`: Use metrics from 1 minute ago until now to calculate that average.
- `percentage`: Clarify that we're calculating a percentage of RAM usage.
- `of used`: Specify which dimension (`used`) on the `system.ram` chart you want to monitor with this entity.
- `units`: Use percentages rather than absolute units.
- `every`: How often to perform the `lookup` calculation to decide whether to trigger this alert.
- `warn`/`crit`: The value at which Netdata should trigger a warning or critical alert. This example uses simple
syntax, but most pre-configured health entities use
[hysteresis](#special-use-of-the-conditional-operator) to avoid superfluous notifications.
- `info`: A description of the alert, which will appear in the dashboard and notifications.
In human-readable format:
> This health entity, named **ram_usage**, watches the **system.ram** chart. It looks up the last **1 minute** of
> metrics from the **used** dimension and calculates the **average** of all those metrics in a **percentage** format,
> using a **% unit**. The entity performs this lookup **every minute**.
>
> If the average RAM usage percentage over the last 1 minute is **more than 80%**, the entity triggers a warning alert.
> If the usage is **more than 90%**, the entity triggers a critical alert.
When you finish writing this new health entity, [reload Netdata's health configuration](#reload-health-configuration) to
see it live on the local dashboard or Netdata Cloud.
## Health entity reference
The following reference contains information about the syntax and options of _health entities_, which Netdata attaches
to charts in order to trigger alerts.
### Entity types
There are two entity types: **alarms** and **templates**. They have the same format and feature set—the only difference
is their label.
**Alerts** are attached to specific charts and use the `alarm` label.
**Templates** define rules that apply to all charts of a specific context, and use the `template` label. Templates help
you apply one entity to all disks, all network interfaces, all MySQL databases, and so on.
Alerts have higher precedence and will override templates.
If the `alert` and `template` entities have the same name and are attached to the same chart, Netdata will use `alarm`.
### Entity format
Netdata parses the following lines. Beneath the table is an in-depth explanation of each line's purpose and syntax.
- The `alarm` or `template` line must be the first line of any entity.
- The `on` line is **always required**.
- The `every` line is **required** if not using `lookup`.
- Each entity **must** have at least one of the following lines: `lookup`, `calc`, `warn`, or `crit`.
- A few lines use space-separated lists to define how the entity behaves. You can use `*` as a wildcard or prefix with
`!` for a negative match. Order is important, too! See our [simple patterns docs](/src/libnetdata/simple_pattern/README.md) for
more examples.
- Lines terminated by a `\` are spliced together with the next line. The backslash is removed and the following line is
joined with the current one. No space is inserted, so you may split a line anywhere, even in the middle of a word.
This comes in handy if your `info` line consists of several sentences.
| line | required | functionality |
|-----------------------------------------------------|-----------------|---------------------------------------------------------------------------------------|
| [`alarm`/`template`](#alert-line-alarm-or-template) | yes | Name of the alert/template. |
| [`on`](#alert-line-on) | yes | The chart this alert should attach to. |
| [`class`](#alert-line-class) | no | The general alert classification. |
| [`type`](#alert-line-type) | no | What area of the system the alert monitors. |
| [`component`](#alert-line-component) | no | Specific component of the type of the alert. |
| [`lookup`](#alert-line-lookup) | yes | The database lookup to find and process metrics for the chart specified through `on`. |
| [`calc`](#alert-line-calc) | yes (see above) | A calculation to apply to the value found via `lookup` or another variable. |
| [`every`](#alert-line-every) | no | The frequency of the alert. |
| [`green`/`red`](#alert-lines-green-and-red) | no | Set the green and red thresholds of a chart. |
| [`warn`/`crit`](#alert-lines-warn-and-crit) | yes (see above) | Expressions evaluating to true or false, and when true, will trigger the alert. |
| [`to`](#alert-line-to) | no | A list of roles to send notifications to. |
| [`exec`](#alert-line-exec) | no | The script to execute when the alert changes status. |
| [`delay`](#alert-line-delay) | no | Optional hysteresis settings to prevent floods of notifications. |
| [`repeat`](#alert-line-repeat) | no | The interval for sending notifications when an alert is in WARNING or CRITICAL mode. |
| [`options`](#alert-line-options) | no | Add an option to not clear alerts. |
| [`host labels`](#alert-line-host-labels) | no | Restrict an alert or template to a list of matching labels present on a host. |
| [`chart labels`](#alert-line-chart-labels) | no | Restrict an alert or template to a list of matching labels present on a chart. |
| [`summary`](#alert-line-summary) | no | A brief description of the alert. |
| [`info`](#alert-line-info) | no | A longer text field that provides more information of this alert |
The `alarm` or `template` line must be the first line of any entity.
#### Alert line `alarm` or `template`
This line starts an alert or template based on the [entity type](#entity-types) you're interested in creating.
**Alert:**
```yaml
alarm: NAME
```
**Template:**
```yaml
template: NAME
```
`NAME` can be any alpha character, with `.` (period) and `_` (underscore) as the only allowed symbols, but the names
cannot be `chart name`, `dimension name`, `family name`, or `chart variables names`.
#### Alert line `on`
This line defines the chart this alert should attach to.
**Alerts:**
```yaml
on: CHART
```
The value `CHART` should be the unique ID or name of the chart you're interested in, as shown on the dashboard. In the
image below, the unique ID is `system.cpu`.
**Template:**
```yaml
on: CONTEXT
```
The value `CONTEXT` should be the context you want this template to attach to.
Need to find the context? Hover over the date on any given chart and look at the tooltip. In the image below, which
shows a disk I/O chart, the tooltip reads: `proc:/proc/diskstats, disk.io`.
You're interested in what comes after the comma: `disk.io`. That's the name of the chart's context.
If you create a template using the `disk.io` context, it will apply an alert to every disk available on your system.
#### Alert line `class`
This indicates the type of error (or general problem area) that the alert or template applies to. For example, `Latency` can be used for alerts that trigger on latency issues on network interfaces, web servers, or database systems. Example:
```yaml
class: Latency
```
Netdata's stock alerts use the following `class` attributes by default:
| Class |
|-------------|
| Errors |
| Latency |
| Utilization |
| Workload |
`class` will default to `Unknown` if the line is missing from the alert configuration.
#### Alert line `type`
Type can be used to indicate the broader area of the system that the alert applies to. For example, under the general `Database` type, you can group together alerts that operate on various database systems, like `MySQL`, `CockroachDB`, `CouchDB` etc. Example:
```yaml
type: Database
```
Netdata's stock alerts use the following `type` attributes by default, but feel free to adjust for your own requirements.
| Type | Description |
|-----------------|------------------------------------------------------------------------------------------------|
| Ad Filtering | Services related to Ad Filtering (like pi-hole) |
| Certificates | Certificates monitoring related |
| Cgroups | Alerts for cpu and memory usage of control groups |
| Computing | Alerts for shared computing applications (e.g. boinc) |
| Containers | Container related alerts (e.g. docker instances) |
| Database | Database systems (e.g. MySQL, PostgreSQL, etc) |
| Data Sharing | Used to group together alerts for data sharing applications |
| DHCP | Alerts for dhcp related services |
| DNS | Alerts for dns related services |
| Kubernetes | Alerts for kubernetes nodes monitoring |
| KV Storage | Key-Value pairs services alerts (e.g. memcached) |
| Linux | Services specific to Linux (e.g. systemd) |
| Messaging | Alerts for message passing services (e.g. vernemq) |
| Netdata | Internal Netdata components monitoring |
| Other | When an alert doesn't fit in other types. |
| Power Supply | Alerts from power supply related services (e.g. apcupsd) |
| Search engine | Alerts for search services (e.g. elasticsearch) |
| Storage | Class for alerts dealing with storage services (storage devices typically live under `System`) |
| System | General system alerts (e.g. cpu, network, etc.) |
| Virtual Machine | Virtual Machine software |
| Web Proxy | Web proxy software (e.g. squid) |
| Web Server | Web server software (e.g. Apache, ngnix, etc.) |
| Windows | Alerts for monitor of windows services |
If an alert configuration is missing the `type` line, its value will default to `Unknown`.
#### Alert line `component`
Component can be used to narrow down what the previous `type` value specifies for each alert or template. Continuing from the previous example, `component` might include `MySQL`, `CockroachDB`, `MongoDB`, all under the same `Database` type. Example:
```yaml
component: MySQL
```
As with the `class` and `type` line, if `component` is missing from the configuration, its value will default to `Unknown`.
#### Alert line `lookup`
This line makes a database lookup to find a value. This result of this lookup is available as `$this`.
The format is:
```yaml
lookup: METHOD(GROUPING OPTIONS) AFTER [at BEFORE] [every DURATION] [OPTIONS] [of DIMENSIONS]
```
The full [database query API](/src/web/api/queries/README.md) is supported. In short:
- `METHOD` is one of the available [grouping methods](/src/web/api/queries/README.md#grouping-methods) such as `average`, `min`, `max` etc.
This is required.
- `GROUPING OPTIONS` are optional and can have the form `CONDITION VALUE`, where `CONDITION` is `!=`, `=`, `<=`, `<`, `>`, `>=` and `VALUE` is a number. The `CONDITION` and `VALUE` are required for `countif`, while `VALUE` is used by `percentile`, `trimmed_mean` and `trimmed_median`.
- `AFTER` is a relative number of seconds, but it also accepts a single letter for changing
the units, like `-1s` = 1 second in the past, `-1m` = 1 minute in the past, `-1h` = 1 hour
in the past, `-1d` = 1 day in the past. You need a negative number (i.e. how far in the past
to look for the value). **This is required**.
- `at BEFORE` is by default 0 and is not required. Using this you can define the end of the
lookup. So data will be evaluated between `AFTER` and `BEFORE`.
- `every DURATION` sets the updated frequency of the lookup (supports single letter units as
above too).
- `OPTIONS` is a space separated list of `percentage`, `absolute`, `min`, `max`, `average`, `sum`,
`min2max`, `unaligned`, `match-ids`, `match-names`.
- `percentage` during time-aggregation, calculate the percentage of the selected dimensions over the total of all dimensions.
- `absolute` during time-aggregation, turns all sample values positive before using them.
- `min` after time-aggregation of each dimension, return the minimum of all dimensions.
- `max` after time-aggregation of each dimension, return the maximum of all dimensions.
- `average` after time-aggregation of each dimension, return the average of all dimensions.
- `sum` after time-aggregation of each dimension, return the sum of all dimensions (this is the default).
- `min2max` after time-aggregation of each dimension, return the delta between the min and the max of the dimensions.
- `unaligned` prevents shifting the query window to multiples of the query duration.
- `match-ids` matches the dimensions based on their IDs (the default is enabled, give `match-names` to disable).
- `match-names` matches the dimension based on their names (the default is enabled, give `match-ids` to disable).
- `of DIMENSIONS` is optional and has to be the last parameter. Dimensions have to be separated
by `,` or `|`. The space characters found in dimensions will be kept as-is (a few dimensions
have spaces in their names). This accepts Netdata simple patterns _(with `words` separated by
`,` or `|` instead of spaces)_ and the `match-ids` and `match-names` options affect the searches
for dimensions.
The result of the lookup will be available as `$this` and `$NAME` in expressions.
The timestamps of the timeframe evaluated by the database lookup is available as variables
`$after` and `$before` (both are unix timestamps).
#### Alert line `calc`
A `calc` is designed to apply some calculation to the values or variables available to the entity. The result of the
calculation will be made available at the `$this` variable, overwriting the value from your `lookup`, to use in warning
and critical expressions.
When paired with `lookup`, `calc` will perform the calculation just after `lookup` has retrieved a value from Netdata's
database.
You can use `calc` without `lookup` if you are using [other available variables](#variables).
The `calc` line uses [expressions](#expressions) for its syntax.
```yaml
calc: EXPRESSION
```
#### Alert line `every`
Sets the update frequency of this alert. This is the same to the `every DURATION` given
in the `lookup` lines.
Format:
```yaml
every: DURATION
```
`DURATION` accepts `s` for seconds, `m` is minutes, `h` for hours, `d` for days.
#### Alert lines `green` and `red`
Set the green and red thresholds of a chart. Both are available as `$green` and `$red` in expressions. If multiple
alerts define different thresholds, the ones defined by the first alert will be used. Eventually it will be visualized
on the dashboard, so only one set of them is allowed If you need multiple sets of them in different alerts, use
absolute numbers instead of `$red` and `$green`.
Format:
```yaml
green: NUMBER
red: NUMBER
```
#### Alert lines `warn` and `crit`
Define the expression that triggers either a warning or critical alert. These are optional, and should evaluate to
either true or false (or zero/non-zero).
The format uses Netdata's [expressions syntax](#expressions).
```yaml
warn: EXPRESSION
crit: EXPRESSION
```
#### Alert line `to`
This will be the first script parameter that will be executed when the alert changes its status. Its meaning is left up to
the `exec` script.
The default `exec` script, `alarm-notify.sh`, uses this field as a space separated list of roles, which are then
consulted to find the exact recipients per notification method.
Format:
```yaml
to: ROLE1 ROLE2 ROLE3 ...
```
#### Alert line `exec`
Script to be executed when the alert status changes.
Format:
```yaml
exec: SCRIPT
```
The default `SCRIPT` is Netdata's `alarm-notify.sh`, which supports all the notifications methods Netdata supports,
including custom hooks.
#### Alert line `delay`
This is used to provide optional hysteresis settings for the notifications, to defend against notification floods. These
settings do not affect the actual alert - only the time the `exec` script is executed.
Format:
```yaml
delay: [[[up U] [down D] multiplier M] max X]
```
- `up U` defines the delay to be applied to a notification for an alert that raised its status
(i.e. CLEAR to WARNING, CLEAR to CRITICAL, WARNING to CRITICAL). For example, `up 10s`, the
notification for this event will be sent 10 seconds after the actual event. This is used in
hope the alert will get back to its previous state within the duration given. The default `U`
is zero.
- `down D` defines the delay to be applied to a notification for an alert that moves to lower
state (i.e. CRITICAL to WARNING, CRITICAL to CLEAR, WARNING to CLEAR). For example, `down 1m`
will delay the notification by 1 minute. This is used to prevent notifications for flapping
alerts. The default `D` is zero.
- `multiplier M` multiplies `U` and `D` when an alert changes state, while a notification is
delayed. The default multiplier is `1.0`.
- `max X` defines the maximum absolute notification delay an alert may get. The default `X`
is `max(U * M, D * M)` (i.e. the max duration of `U` or `D` multiplied once with `M`).
Example:
`delay: up 10s down 15m multiplier 2 max 1h`
The time is `00:00:00` and the status of the alert is CLEAR.
| time of event | new status | delay | notification will be sent | why |
|---------------|------------|---------------------|---------------------------|-------------------------------------------------------------------------------|
| 00:00:01 | WARNING | `up 10s` | 00:00:11 | first state switch |
| 00:00:05 | CLEAR | `down 15m x2` | 00:30:05 | the alert changes state while a notification is delayed, so it was multiplied |
| 00:00:06 | WARNING | `up 10s x2 x2` | 00:00:26 | multiplied twice |
| 00:00:07 | CLEAR | `down 15m x2 x2 x2` | 00:45:07 | multiplied 3 times. |
So:
- `U` and `D` are multiplied by `M` every time the alert changes state (any state, not just
their matching one) and a delay is in place.
- All are reset to their defaults when the alert switches state without a delay in place.
#### Alert line `repeat`
Defines the interval between repeating notifications for the alerts in CRITICAL or WARNING mode. This will override the
default interval settings inherited from health settings in `netdata.conf`. The default settings for repeating
notifications are `default repeat warning = DURATION` and `default repeat critical = DURATION` which can be found in
health stock configuration, when one of these interval is bigger than 0, Netdata will activate the repeat notification
for `CRITICAL`, `CLEAR` and `WARNING` messages.
Format:
```yaml
repeat: [off] [warning DURATION] [critical DURATION]
```
- `off`: Turns off the repeating feature for the current alert. This is effective when the default repeat settings has
been enabled in health configuration.
- `warning DURATION`: Defines the interval when the alert is in WARNING state. Use `0s` to turn off the repeating
notification for WARNING mode.
- `critical DURATION`: Defines the interval when the alert is in CRITICAL state. Use `0s` to turn off the repeating
notification for CRITICAL mode.
#### Alert line `options`
The only possible value for the `options` line is
```yaml
options: no-clear-notification
```
For some alerts we need compare two time-frames, to detect anomalies. For example, `health.d/httpcheck.conf` has an
alert template called `web_service_slow` that compares the average http call response time over the last 3 minutes,
compared to the average over the last hour. It triggers a warning alert when the average of the last 3 minutes is twice
the average of the last hour. In such cases, it is easy to trigger the alert, but difficult to tell when the alert is
cleared. As time passes, the newest window moves into the older, so the average response time of the last hour will keep
increasing. Eventually, the comparison will find the averages in the two time-frames close enough to clear the alert.
However, the issue was not resolved, it's just a matter of the newer data "polluting" the old. For such alerts, it's a
good idea to tell Netdata to not clear the notification, by using the `no-clear-notification` option.
#### Alert line `host labels`
Defines the list of labels present on a host. See our [host labels guide](/docs/netdata-agent/configuration/organize-systems-metrics-and-alerts.md) for
an explanation of host labels and how to implement them.
For example, let's suppose that `netdata.conf` is configured with the following labels:
```yaml
[host labels]
installed = 20191211
room = server
```
And more labels in `netdata.conf` for workstations:
```yaml
[host labels]
installed = 201705
room = workstation
```
By defining labels inside of `netdata.conf`, you can now apply labels to alerts. For example, you can add the following
line to any alerts you'd like to apply to hosts that have the label `room = server`.
```yaml
host labels: room = server
```
The `host labels` is a space-separated list that accepts simple patterns. For example, you can create an alert
that will be applied to all hosts installed in the last decade with the following line:
```yaml
host labels: installed = 201*
```
See our [simple patterns docs](/src/libnetdata/simple_pattern/README.md) for more examples.
#### Alert line `chart labels`
Similar to host labels, the `chart labels` key can be used to filter if an alert will load or not for a specific chart, based on
whether these chart labels match or not.
The list of chart labels present on each chart can be obtained from
For example, each `disk_space` chart defines a chart label called `mount_point` with each instance of this chart having
a value there of which mount point it monitors.
If you have an e.g. external disk mounted on `/mnt/disk1` and you don't wish any related disk space alerts running for
it (but you do for all other mount points), you can add the following to the alert's configuration:
```yaml
chart labels: mount_point=!/mnt/disk1 *
```
The `chart labels` is a space-separated list that accepts simple patterns. If you use multiple different chart labels,
then the result is an AND between them. i.e. the following:
```yaml
chart labels: mount_point=/mnt/disk1 device=sda
```
Will create the alert if the `mount_point` is `/mnt/disk1` and the `device` is `sda`. Furthermore, if a chart label name
is specified that does not exist in the chart, the chart won't be matched.
See our [simple patterns docs](/src/libnetdata/simple_pattern/README.md) for more examples.
#### Alert line `summary`
The summary field contains a brief title of the alert. It is used as the subject for the notifications, and in
dashboard list of alerts. An example for the `ram_available` alert is:
```yaml
summary: Available Ram
```
summary fields can contain special variables in their text that will be replaced during run-time to provide more specific
alert information. Current variables supported are:
| variable | description |
|---------------------|-------------------------------------------------------------------|
| ${family} | Will be replaced by the family instance for the alert (e.g. eth0) |
| ${label:LABEL_NAME} | The variable will be replaced with the value of the chart label |
For example, a summary field like the following:
```yaml
summary: 1 minute received traffic overflow for ${label:device}
```
Will be rendered on the alert acting on interface `eth0` as:
```yaml
summary: 1 minute received traffic overflow for eth0
```
> Please note that variable names are case-sensitive.
#### Alert line `info`
The info field can contain a small piece of text describing the alert or template. This will be rendered in
notifications and UI elements whenever the specific alert is in focus. An example for the `ram_available` alert is:
```yaml
info: Percentage of estimated amount of RAM available for userspace processes, without causing swapping
```
info fields can contain special variables in their text that will be replaced during run-time to provide more specific
alert information. Current variables supported are:
| variable | description |
|---------------------|-------------------------------------------------------------------|
| ${family} | Will be replaced by the family instance for the alert (e.g. eth0) |
| ${label:LABEL_NAME} | The variable will be replaced with the value of the chart label |
For example, an info field like the following:
```yaml
info: average inbound utilization for the network interface ${family} over the last minute
```
Will be rendered on the alert acting on interface `eth0` as:
```yaml
info: average inbound utilization for the network interface eth0 over the last minute
```
An alert acting on a chart that has a chart label named e.g. `target`, with a value of `https://netdata.cloud/`,
can be enriched as follows:
```yaml
info: average ratio of HTTP responses with unexpected status over the last 5 minutes for the site ${label:target}
```
Will become:
```yaml
info: average ratio of HTTP responses with unexpected status over the last 5 minutes for the site https://netdata.cloud/
```
> Please note that variable names are case-sensitive.
## Expressions
Netdata has an internal infix expression parser under `libnetdata/eval`. This parses expressions and creates an internal
structure that allows fast execution of them.
These operators are supported `+`, `-`, `*`, `/`, `<`, `==`, `<=`, `<>`, `!=`, `>`, `>=`, `&&`, `||`, `!`, `AND`, `OR`, `NOT`.
Boolean operators result in either `1` (true) or `0` (false).
The conditional evaluation operator `?` is supported too. Using this operator IF-THEN-ELSE conditional statements can be
specified. The format is: `(condition) ? (true expression) : (false expression)`. So, Netdata will first evaluate the
`condition` and based on the result will either evaluate `true expression` or `false expression`.
Example: `($this > 0) ? ($avail * 2) : ($used / 2)`.
Nested such expressions are also supported (i.e. `true expression` and `false expression` can contain conditional
evaluations).
Expressions also support the `abs()` function.
Expressions can have variables. Variables start with `$`. Check below for more information.
There are two special values you can use:
- `nan`, for example `$this != nan` will check if the variable `this` is available. A variable can be `nan` if the
database lookup failed. All calculations (i.e. addition, multiplication, etc.) with a `nan` result in a `nan`.
- `inf`, for example `$this != inf` will check if `this` is not infinite. A value or variable can be set to infinite
if divided by zero. All calculations (i.e. addition, multiplication, etc.) with a `inf` result in a `inf`.
### Special use of the conditional operator
A common (but not necessarily obvious) use of the conditional evaluation operator is to provide
[hysteresis](https://en.wikipedia.org/wiki/Hysteresis) around the critical or warning thresholds. This usage helps to
avoid bogus messages resulting from small variations in the value when it is varying regularly but staying close to the
threshold value, without needing to delay sending messages at all.
An example of such usage from the default CPU usage alerts bundled with Netdata is:
```yaml
warn: $this > (($status >= $WARNING) ? (75) : (85))
crit: $this > (($status == $CRITICAL) ? (85) : (95))
```
The above say:
- If the alert is currently a warning, then the threshold for being considered a warning is 75, otherwise it's 85.
- If the alert is currently critical, then the threshold for being considered critical is 85, otherwise it's 95.
Which in turn, results in the following behavior:
- While the value is rising, it will trigger a warning when it exceeds 85, and a critical alert when it exceeds 95.
- While the value is falling, it will return to a warning state when it goes below 85, and a normal state when it goes
below 75.
- If the value is constantly varying between 80 and 90, then it will trigger a warning the first time it goes above
85, but will remain a warning until it goes below 75 (or goes above 85).
- If the value is constantly varying between 90 and 100, then it will trigger a critical alert the first time it goes
above 95, but will remain a critical alert goes below 85 (at which point it will return to being a warning).
## Variables
You can find all the variables that can be used for a given chart, using
`http://NODE:19999/api/v1/alarm_variables?chart=CHART_NAME`, replacing `NODE` with the IP address or hostname for your
Agent dashboard. For example, [variables for the `system.cpu` chart of the
registry](https://registry.my-netdata.io/api/v1/alarm_variables?chart=system.cpu).
Netdata supports 3 internal indexes for variables that will be used in health monitoring.
The variables below can be used in both chart alerts and context templates.
Although the `alarm_variables` link shows you variables for a particular chart, the same variables can also be used in
templates for charts belonging to a given context. The reason is that all charts of a given
context are essentially identical, with the only difference being the family that identifies a particular hardware or software instance.